The invention relates a system for generating a signal on a vehicle, specifically in conjunction with an undercarriage control.
An efficient suspension system between the wheel units and the vehicle superstructure is essential for the design of the undercarriage of a motor vehicle. In the case of a semiactive system, such a suspension system consists generally of a spring arrangement having a fixed spring characteristic with which a shock absorber device with adjustable damping is arranged in parallel thereto. Such a shock absorber with an adjustable characteristic may be realized in such a way that the shock absorber system is equipped with a flow control valve whose cross sectional flow is variable.
Furthermore, an efficient process for the control or regulation of the adjustable suspension system is of great importance for the design of such an undercarriage. Such a process, based on information from sensor signals which apprise the state of travel of the vehicle, provides activation signals for the adjustable suspension systems.
An efficient undercarriage regulation or control should ideally regulate or control the adjustable undercarriage in such a way that, for one, it allows for road safety and, for another, enables maximum travel comfort for the passengers and/or a vehicle cargo that is sensitive to shocks. From the aspect of the suspension and/or shock absorber system, these are conflicting objectives. High travel comfort can be achieved through a maximally soft undercarriage adjustment, whereas with regard to high road safety, a maximally hard undercarriage adjustment is desirable.
Previously known from DE-OS 39 18 735 is a process for damping sequences of motion on undercarriages of passenger cars and trucks. Here, the activation signals for control or regulation of the adjustable undercarriage are essentially generated by the processing of sensor signals through filter setups. These filters are so conceived that the sensor signals which apprise the travel state of the vehicle will be influenced in their amplitude and/or phase development. This influencing generates activation signals for the adjustable undercarriage, thereby effecting an adaptation to the respective state of movement of the vehicle in such a way that in critical travel situations an undercarriage adjustment serving the road safety mode will be brought about while in uncritical travel situations an adjustment for comfort will be made.
An undercarriage comfort adjustment can be accomplished in that the adjustable undercarriage is adjusted maximally soft, i.e., such that the adjustable shock absorbers exercise a slight damping. A far more efficient control or regulation of the undercarriage, for example in view of the movements of the vehicle superstructure that determine travel comfort, can be accomplished through a so-called frequency-dependent "skyhook regulation."
With the so-called skyhook regulation, the superstructure movements are reduced so as to bring about an improvement of the travel comfort, whereas the road safety is not directly increased. Generally known in undercarriage control, this concept of regulation is based on the model concept of a shock absorber and/or suspension system that attaches to the mass of the vehicle superstructure and is connected to an inertial fix point (skyhook). Such an inertial shock absorber and/or suspension system not being directly realizable in practice, is by way of substitution, appropriately activated between the vehicle superstructure and the wheel units.
There is known from a number of publications (Crolla, D. A., Aboul Nour, A.M.A., Proceedings of the Institution of Mechanical Engineers, International Conference of Advanced Suspension, Oct. 22-25, 1988, London or Magolis, D. L., Semi-Active Heave and Pitch Control for Ground Vehicles, Vehicle System Dynamics, 11 (1982), pp. 31-42), in the case of a suspension system featuring shock absorbers whose damping characteristic is adjustable in two stages (hard/soft), "semiactive, discrete skyhook damping" which is a switching strategy wherein the damping characteristic is adjusted contingent on superstructure movements. This strategy is presented in the following table:
______________________________________ Shock absorber Shock absorber in pull state in push state ______________________________________ Va &gt; Vagr hard soft Vagr soft hard ______________________________________
Here, the superstructure velocity in the vertical direction, at the points of attack of the suspension system, is abbreviated as Va. Once this velocity exceeds a certain positive bound Vagr (tuning parameter), i.e., as an impetuous upward movement of the car body is taking place, the respective shock absorber is switched in the pull state to the hard characteristic, and in the push state to the soft characteristic. Conversely, an impetuous down movement of the superstructure causes in the pull stage, a changeover to the soft and in the push stage to the hard characteristic. In the absence of excessive superstructure movements (.vertline.Va.vertline..ltoreq.Vagr) the shock absorber operates in its soft tuning mode, both in the pull and the push stage.
Shock absorbers that are adjustable in their damping characteristic are described in DE-OS 33 04 815 and DE-OS 36 44 447.
Furthermore, thoughts concerning the road safety are relevant as criteria for adjustment of the damping characteristic. Geared to minimizing the dynamic wheel load fluctuations, such a system has been described in the German patent application P 40 11 808.8.
Such undercarriage regulation systems provide control signals for the adjustment of the shock absorber damping characteristic contingent upon the state of travel of the vehicle.
U.S. Pat. No. 4,936,925 proposes an undercarriage regulation system wherein a changeover of a semiactive shock absorber is to take place between a hard and a soft damping stage whenever the relative velocity of the two points of attack of the shock absorbers is less than a fixedly predetermined thresh-old or the tire deformation is smaller than a fixedly predetermined thresh-old, depending on which of the two conditions of changeover is first met. As will be shown hereinbelow, such an activation contingent on shock absorber piston velocity is not optimal. Even when additionally allowing for the tire deformation as a criterion for the changeover of the damper characteristic, an optimal mode of activation is not obtained, as will be described hereinbelow. Furthermore, allowing for the tire deformation requires a considerable expense with regard to the sensor engineering needed.
The problem underlying the present invention, based on these activation signals, is to optimize the mode of activation of the shock absorbers.